The use of animal viruses as model systems for probing the complexities of molecular control mechanisms has been particularly fruitful. It is generally throught that the understanding of genetic regulation in viruses will provide an insight into similar processes in eucaryotic cells. The overall objective of our research is to systematically develop our understanding of gene regulation in virus infected cells. This knowledge will be used for studying the regulation of normal cellular genes and oncogenes and for developing in vitro systems from which activities can be purified and characterized. In the present revised renewal application we especially intend to accomplish the following: 1. To characterize the structure of actively transcribed SV40 minichromosomes. 2. To study the mechanism of transcription termination at the attenuation site. 3. To study attenuation as a mechanism regulating viral and cellular gene expression (globin and c-myc). 4. To purify and characterize cellular termination and readthrough factors. 5. To study the significance of RNA secondary structure in the regulation of eucaryotic gene expression. 6. To study the role of agnoprotein in regulating SV40 gene expression and in virus assembly. The acquisition of knowledge by the present research will ultimately contribute to a better understanding of the more complex phenomena in mammalian cells, such as the molecular processes of differentiation, development and malignant transformation.